1. Field of the Invention
This invention relates to a low-density resin impregnated ceramic article and method for making the same. The resin impregnated ceramic article is advantageously employed as a structural ceramic ablator for use, e.g., in a thermal protection system (TPS) such as an ablative heat shield for a high speed atmospheric entry vehicle. This novel material is structurally stable, and can be easily tailored to accommodate specific mission requirements. Other applications of the inventive resin impregnated ceramic article and method for making the same include fire retardant structures, a reusable thermal protection system for the next generation Space Shuttle (X-2000), light weight structural components for the automotive and space industries, and water proofing for a TPS or external structures of a space vehicle.
2. Description of the Prior Art
Ablative polymers and polymeric composites serve an important function in aerospace technology. They protect aerodynamic surfaces, propulsion structures, and ground equipment from the degradative effects of very high temperatures or incident heating rates. This protective function is accomplished by a self regulating heat and mass transfer process known as ablation.
High density ablators having a density of about 1.1-1.9 g/cc (68.6-118 lb.sub.m /ft.sup.3) have been developed from various polymers such as epoxy, phenolic resins and different reinforced fibers or fillers such as asbestos fibers, graphite cloth, silica cloth, etc. by mixing, pressing and heating processes.
Existing low density ablators such as SLA-561 and AVCOAT (described in Bartlett, E. P. and Andersen, L. W., "An Evaluation of Ablation Mechanism for the Apollo Heat Shield Material", Aerotherm Report No 68-38, Part II, Oct. 15, 1968) and MA-25S (described in Williams, S. D., "Thermophysical Properties Used for Ablation Analysis", LEC-13999, December 1979) are made of polymers, silica or phenolic microballons and are filled with chopped fibers and/or a honeycomb structure for reinforcement. These conventional ablators (developed and manufactured by Martin Marietta) have been successfully used in many planetary missions; for example, AVCOAT 5026-39HCG was used in a TPS for the Apollo capsules, SLA-561V was used on the Viking probe, and MA-25S is used on the nose cone of the Space Shuttle external fuel tank. The principal heat protection of these ablators is provided by the polymer and high level of carbon char formed as a residue during the ablation process. The heat dissipation process is due to heat absorption from depolymerization and gas pyrolysis, re-radiation from the char layer, and convective heat blockage from pyrolysis gas blowing in the boundary layer. These ablators have been extensively characterized and are flight verified.
During the early years of Space Shuttle heat shield development, a passive transpiration system was proposed. The system included a low density, high temperature ceramic matrix, such as silica, carbon, potassium titanate, or graphite, impregnated with coolants such as polyethylene, epoxy, acrylic, and phenolic. Of the different candidates, only the silica fiber matrix was studied in detail, mainly using LI-1500 material as described in R. P. Banas et al., "Lifting Entry Vehicle Thermal Protection Review" Lockhead Report No. 4-83-4-2 (1964). The silica fibrous matrix was totally immersed in the melted organic coolant, resulting in the coolant occupying all the void volume of the matrix. The ceramic ablators thus prepared had densities in the range of about 1.0-1.3 g/cc (60-80 lb.sub.m /ft.sup.3); 78% by weight as the organic coolant and 22% by weight as the silica fibers.
The passive transpiration system described above increases the heat rate capability of the ceramic substrate by addition of an organic coolant which functions as a transpirant. The disadvantage of this system is that the high density of the final product increases the overall thermal conductivity of the system. In addition, because the organic coolant entirely fills the void volume of the ceramic matrix, the organic coolant acts as an effective conduction path.
Conventional ablators are generally manufactured in a single process in which the polymers and other components, such as the microballons and the reinforced fibers, are uniformly mixed and allowed to cure. The final products have a uniform density which would be a disadvantage in economizing the vehicle's weight. One improvement in accordance with a preferred embodiment of this invention is to decrease the overall TPS weight by having a density gradient along the heat shield thickness, e.g., high density at the outer surface where needed and very low density near the vehicle structure. For conventional ablators, this modification would require special attachment schemes of different individual layers of the ablators that might be costly.
Silicone elastomeric ablators employing a polymer resin as the main structural support component for the entire heat shield have a fairly low density of from 0.2-0.4 g/cc (14-25 lb.sub.m ft.sup.3); however, these materials have very low mechanical strength. After the decomposition process takes place, the polymeric residue is of very low strength and can be removed by low aerodynamic shear loads. Consequently, these materials are useful only for relatively low heating, low pressure environments of below about 1135 kW/m.sup.2 (100 BTU/s.ft.sup.2). This lack of self-support might cause a change in the vehicle's design shape that would ultimately affect the aerodynamic flow regime about the vehicle. Other ablators such as AVCOAT retain structural integrity with the help of a reinforcing honeycomb. However, the overall density of these materials is relatively high (0.5 g/cc-1.0 g/cm or 30-60 lb.sub.m /ft.sup.3), thereby resulting in high overall TPS weights.
The following patents relate to ceramic heat shields and ablative structures.
U.S. Pat. No. 4,713,275 relates to a ceramic tile for use in a thermal protection system, employing a ceramic cloth having additional ceramic material deposited therein. Dual tiles interlock with one another to form a single unit. The inner ceramic is designed to be of lighter weight.
U.S. Pat. No. 4,804,571 relates to a thermal protection system used for reentry vehicles or high speed aircraft including multiple refractory tiles of varying thickness defined by thermal requirements at the point of installation.
U.S. Pat. No. 4,031,059 relates to low-density ablators comprising a siloxane elastomer resin and a low-density filler material including ground cork, silica or glass microspheres and hollow phenolic resin microballons. The ablator may further contain carbon and/or silica fibers.
U.S. Pat. No. 4,100,322 relates to a high thermal capacity fiber-resin-carbon composite ablator having a polymer resin filler. The composite is prepared by impregnating a woven fabric of carbon or graphite yarn with a resin, curing the resin, pyrolyzing the impregnated fabric and re-impregnating the resulting fiber-porous carbon char composite with resin.
U.S. Pat. No. 4,605,594 relates to a ceramic article including a woven ceramic cloth having a non-porous core and a porous periphery prepared by treating with an acid. The porous periphery can be infiltrated by any of a number of desired materials such as a metal, a metal oxide, a catalyst and an elastomer. The articles of interest can be used as fiber optic elements, catalyst supports and heat resistant tiles for aerospace purposes.
U.S. Pat. No. 4,743,511 relates to a method of producing a refractory cermet article comprising a continuous ceramic phase and a discontinuous metal phase.
U.S. Pat. No. 5,112,545 relates to a composite preform prepared by first impregnating a braided preform of inorganic filaments with a silicone resin solution. The impregnated preform is heated at a first temperature under pressure to cure the resin and then heated at a second higher temperature to convert the cured resin to silica solids that are dispersed between the strands of the preform. The rigidized preform is then infiltrated with an inorganic matrix such as a ceramic material.
U.S. Pat. No. 5,154,787 describes a method of manufacturing prepreg mats. Therein, a prepreg strand formed of inorganic fibers impregnated with a thermoplastic binder or a ceramic matrix powder is heated, cooled and compacted to fuse the same into a uniform, dense prepreg mat.
U.S. Pat. No. 3,533,813 relates to a low density, non-structural ceramic employing a porous ceramic support in combination with organic fillers. One step of the process described therein is the burning off of the organics to form pores within a ceramic. This reduces the mass of the composite, thereby reducing its density while maintaining inherent strength.
U.S. Pat. No. 4,255,197 relates to refractory products and a process for controlling porosity and density. Polymer substrate particles are wetted and mixed with ceramic particles to form a composite, and the composite is heated to burn out the polymer to thereby leave voids.
U.S. Pat. No. 4,828,774 relates to a process for preparing a porous ceramic refractory material for use in aerospace applications. The refractory material is made by firing a felt prepared from a mixture of a ceramic polymer solution and high strength fibers.
U.S. Pat. No. 5,006,492 relates to a low-density silicon nitride ceramic composition used to make flexible, low-density refractory products.
U.S. Pat. No. 5,135,691 relates to a low temperature sintering process for preparing smooth ceramic products. A porous ceramic is infiltrated with an inorganic polymer such as polyaluminoxane. Low temperature heat is applied, which results in a smooth, strong refractory.
U.S. Pat. No. 5,167,271 relates to a method for forming a dense ceramic-metal matrix article, including pyrolizing a ceramic powder-organic compact and infiltrating the resulting porous ceramic preform with molten metal.
U.S. Pat. No. 3,138,009 relates to a transpiration cooling system for use in cooling aircraft operating at hypersonic speeds. Specifically, a transpiration system is shown consisting of a porous skin through which a fluid is forced. Heat levels are reduced as the fluid is vaporized.
U.S. Pat. No. 3,213,166 relates to preparation of porous ceramics for thermal barrier or refractory applications using an organic air cell forming and sustaining compound.
U.S. Pat. No. 3,243,313 relates to a nose cone construction of layers of differing materials including ceramic and metallic materials which are either thermally matched or structurally compensated to prevent delamination.
U.S. Pat. No. 3,533,813 relates to a process for preparation of low-density, high strength ceramic tiles employing a combustible organic filler to provide pores in the final product.
U.S. Pat. No. 4,456,208 relates to a two-piece thermal tile having differing thermal and mechanical characteristics in the inner and outer layers of the tile.